1. Field of the Invention
This invention relates to liquid level detection systems, and more particularly, to a system to detect the liquid level in a server container of a beverage brewing apparatus adapted for use on aircraft.
2. General Background and State of the Art
One conventional technique for determining the level of a liquid in a vessel or container involves mechanically measuring the liquid level with a contact device such as a float indicator, dip stick or probe which extends intrusively into the liquid to be measured. When such techniques are used for measuring liquid level in a server container of a beverage brewing apparatus, such as a coffee brewing machine, these techniques require a sensing device to extend into the server container to contact the liquid, which subjects the sensing device to damage when the server container is repetitively installed and removed from the beverage maker brewing cavity.
One non-contact liquid level sensing technique for non-intrusively detecting the level of the surface of a fluid in a vessel utilizes a laser liquid level gauge that includes an optical unit in the top wall of the vessel. Reflections of an incident laser beam from the fluid boundary are focused by optics to a photodiode producing an output signal received by a control unit, which includes a phase detector for comparing the phase of the modulation tone with that of the detection signal to produce a level indicating signal for display. Another non-contact liquid level sensing technique for measuring the volume of liquid in a container utilizes an ultrasonic system. An ultrasonic signal is emitted from the lid of the container, and is received by a sensor subsystem located on the underside of the lid of the ultrasonic system. The ultrasonic system measures the level of liquid in the container by determining the time the ultrasonic signals take to travel to and return from the surface of the liquid reflecting the ultrasonic signals to the ultrasonic sensor subsystem.
Another liquid level sensing technique utilizes an opto-electronic level sensor including a system with an infrared light emitting diode (IRED) and two silicon phototransistors. A first phototransistor monitors the light reflected from the liquid, and the second phototransistor directly monitors the IRED beam for power degradation or failure and provides interrupt control as required. While the failure of the IRED can be detected directly in this manner, failure of the first phototransistor can go undetected, allowing an overflow to occur. A temperature sensor included with the liquid level sensor that extends intrusively to contact the liquid is also vulnerable to being damaged by repeated usage. Another inherent problem with this approach is that the light emitting and sensing optical sensors are not protected from the steam, water condensation and accumulation of coffee residue, and frequent cleaning operations are required to keep the level sensing system operational, which is time consuming and expensive. Typical cleaning of the liquid level sensors requires inserting a cotton swab moistened with alcohol into the sensor ports, and clearing the sensor ports of lint from the cotton swab. Failure to perform these cleaning operations typically results in uncontrolled overflows due to accumulation of debris and coffee residue on the light emitting and sensing optical sensors. It would therefore be desirable to provide a liquid level sensing device that is less prone to contamination.
Such conventional liquid level sensing systems also typically do not provide any redundancies in the event that the primary sensing apparatus monitoring the liquid level fails, either by internal failure or due to coffee residue build up. Failure to detect the liquid level will result in an overflow, which is not only inconvenient, but also dangerous for the operator. During an overflow, very hot liquid typically comes out of the server and the beverage maker, and spills out, potentially burning the operator attending the machine. It would therefore be desirable to provide a backup liquid level sensing system for preventing accidental overflows in case of a malfunction of a primary liquid level sensing system. The present invention satisfies these and other requirements for aircraft galley systems.